1. Technical Field
This invention relates generally to electromagnetic rail guns, and more particularly to a new and improved launch assembly for use with such a gun.
2. Background Information
Electromagnetic guns are used to accelerate small projectiles to hypervelocity speeds. The projectile structure may include a projectile enclosed by a sabot, and this structure is configured to fit snugly in the gun barrel. Together, the projectile structure and electromagnetic gun may be called an electromagnetic gun assembly that launches the projectile. It does this with electromagnetic forces produced as electric current flows down a conductive rail on one side of the barrel, across a conductive medium (armature) at the rear of the projectile structure, and then back along a second conductive rail on the other side of the barrel.
Due to the high velocities attained, the interface between the sabot and the barrel is subject to large shear or frictional forces as the projectile structure is accelerated down the barrel. The associated frictional heating can lead to excessive ablation of the sabot so that the gap at the sabot-barrel interface increases. Also, barrel expansion from internal pressure and shot-to-shot wear can result in dynamic and static increases in the barrel size relative to the projectile structure. As these effects occur, the increased gap may result in a balloting motion of the projectile structure, i.e. an oscillating lateral motion caused by a loose fit.
This balloting motion is characterized by repetitive ricocheting of the projectile structure off opposite walls of the gun barrel that can result in structural failure of the sabot and excessive damage to the barrel. In addition, balloting can introduce angular tip-off, where lateral forces cause the projectile to exit the muzzle at an angle inclined to the barrel axis.
Consequently, it is desirable to have an electromagnetic gun assembly that maintains the projectile structure in substantially constant and uniform contact with the barrel during launch in order to eliminate such spurious motion.
Prior art in this field includes U.S. Pat. No. 112,121 to Butler which relies on a flexible metallic disk that is flattened against the projectile base by the force of discharge. The motion of the disk forces attached wedges forward into beveled slots in the projectile, thereby driving out radial studs into grooves in the barrel wall. The motion of the studs in the grooves imparts rotation to the projectile. Each wedge requires a pin or key inserted into the studs which prevents the studs from being thrown from the socket after the projectile leaves the gun.
The projectile described in the Butler patent requires a metal base plate, a large number of interrelated moving parts to initiate motion of the studs, keys and pins to retain the studs after exit from the barrel, and it is intended to rotate the projectile. None of these features are required or desired with the present invention. In addition, the present invention is not restricted to projectile structures of circular cross-section, and it does not require grooves (rifling) in the barrel walls.
U.S. Pat. No. 1,098 to James describes a projectile to be used in a rifled cannon for the purpose of imparting to the projectile, when fired, a rotary motion about its axis and to eliminate windage, thereby increasing the force of the discharge. It relies on the pressure in the chamber, or the motion of a tapered wedge in the projectile base to force an expansive packing surrounding the projectile into contact with the bore of the gun and into grooves thereof. This causes the projectile to be rotated as it is being discharged.
The projectile described in the James patent relies on the force of discharge acting directly on the packing or on a conical wedge to expand the packing into rifled grooves. In addition, it is restricted to projectiles of circular cross-section, and it is intended to rotate the projectile.
U.S. Pat. No. 34,493 to Havens describes a projectile requiring two cast iron pieces, and it is restricted to cylindrically-shaped projectiles of circular cross-section. The rear piece has a conical front portion which mates to the base of the front piece. When the charge is fired the rear piece of the projectile is driven toward the front piece with the conical front section causing tapered segments to expand radially against the wall of the gun. This radial motion causes the tapered segments to expand into grooves in the barrel to prevent windage.
Thus the projectile described in the Havens patent requires a two-piece projectile, it relies on relative motion between the front and rear portions of the projectile, and it depends on the force of the explosive charge.
U.S. Pat. No. 3,023,704 to Dawson, et al. describes projectiles for mortars and like projectors having unrifled barrels. Such projectiles require a comparatively large clearance between the largest circumference of the projectile structure and the barrel walls. The projectile is intended to reduce leakage of propellant gases through the clearance gap and is restricted to projectiles of circular cross-section. It relies on the pressure of the propellant gases to distort a resilient annular ring or to move it forward to seal the gap, as opposed to eliminating spurious lateral motion of the projectile.